1. Field of the Invention
This invention pertains to the general field of metal fusion bonding, and in particular to an ultrasonic wedge bonder which is responsive to timed controls. It utilizes an illuminated optical viewing means, utilizing a compliant cushioning medium to grasp and pull a fine soft wire, and uses high frequency vibratory energy to effect a cold weld. It also has means to juxtapose and bond plural workpieces and is a wire lead bonder.
This invention pertains to an improved tailless wire bonder used in the bonding of semiconductor devices to substrates and utilizes adjustable linear actuating devices energized in sequence by a solid state circuit to operate as latching and release mechanisms to control automatically the Z-motion of the bonding arm while also providing for manual control thereof, a wire clamp with compliant jaws, and a hydraulic damping system to control the velocity of the downward movement of the bonding tool and its subsequent mechanical vibration.
2. Description of the Prior Art
In the manufacture of miniature semiconductor electronic devices, one of the most critical steps, in terms of device reliability, is the formation of the electrical interconnections among the various semiconductor chips and the terminals leading to the external circuits. A standard procedure for accomplishing this interconnection is to attach a fine gold or aluminum wire, on the order of one to five mils in diameter, between the metallized pads on the chip itself and the other terminal to which electrical continuity is required. The wire can be attached by a method called wedge bonding, in which the wire is pressed down against the terminal by a metal wedge which is rapidly vibrated across the bonding surface by an attached ultrasonic transducer, such that the combination of pressure and friction created welds the wire to the terminal without the need for additional heat.
Several factors can produce undesirable results in the wedge-bonding operation. Uncontrolled vibration of the wedge can produce inconsistent pressure against the terminal and result in a bond of insufficient strength; it can also snap the fragile wire. Excessive pressure can thin out the wire to such an extent that a break can occur at the junction between the bonded and unbonded portions of the wire. Excessive pressure also can fracture the glass-like semiconductor material beneath the metallized terminal, leading to an eventual device failure. Deformation of the wire by any handling apparatus before it is fed beneath the wedge can cause variations in the surface contact area between wire and terminal, further reducing the effectiveness of the bond, as well as decreasing the wire's cross-sectional area, making it more susceptible to breakage.
In presently existing wedge-bonding machines, the vertical motion of the wedge onto the workpiece is controlled by an arrangement of motors, belts, gears, levers, cams, microswitches or other electromechanical components. U.S. Pat. No. 3,328,875, U.S. Pat. No. 3,444,612 and U.S. Pat. No. 3,643,321 offer examples of such arrangements. However, such mechanisms may initiate undesirable vibrations in the bonding tool which can continue undamped through the precise instant of welding. If the vibration is in the vertical direction, the wedge can bounce in a jackhammer-like fashion on the wire, producing fluctuations in the bonding pressure.
After the second bond, and therefore after the interconnection has been completed, the remaining wire must be separated from the bond so that subsequent bonds may be made. The separation should be made as close as possible to the bonded area to minimize the size of the tail left protruding therefrom.
U.S. Pat. No. 3,626,590 attempts to solve the tail-removal problem by severing the wire before the second bond is made. A length of wire is fed out from the first bond to a position generally above the site of the second bond, it is cut, and then this loose end must be guided downward by the tool in a somewhat intricate maneuver to the proper bonding position. U.S. Pat. No. 3,643,321 and U.S. Pat. No. 3,863,827 provide for tail removal simply by pulling the wire upward, away from the second bond, until it separates at the heel of the bond. However, this procedure tends to lift the bond upward as well, possibly weakening the adhesion to the underlying terminal. This procedure also may require a system to cause the tool to heel or lean toward the remaining wire to provide a deeper bond deformation at the heel, thus assisting the tail pulling and wire breaking procedure. This requires complex mechanical arrangements and provides sources for unwanted motion during the bond.